The present invention relates generally to an apparatus for low temperature refrigeration systems. More particularly, the present invention relates to a non-hydrocarbon design of ultra-low temperature refrigeration systems.
In refrigeration systems, a refrigerant gas is compressed in a compressor unit. Heat generated by the compression is then removed generally by passing the compressed gas through a water or air cooled condenser coil. The cooled, condensed gas is then allowed to rapidly expand into an evaporating coil where the gas becomes much colder, thus cooling the coil and the inside of the refrigeration system box around which the coil is placed.
In the compressor unit one or more chambers are provided which force the refrigerant from an inlet side at a low pressure to an outlet side at a high pressure. Valves prevent the high pressure gas from escaping back to the low pressure side from the high pressure side. The compressor unit will be partially filled with a refrigerant oil to lubricate and seal moving parts. A small amount of this lubricant continuously circulates throughout the refrigeration system
In some multi-stage refrigeration systems, a first refrigeration system will be used to cool the refrigerant gas in a second stage. A problem which arises in these systems is that compressor oil in the second stage evaporating coil or expansion device becomes too viscous thereby inhibiting its ability to flow and return to the compressor.
To combat this problem, small amounts of hydrocarbons (HC) such as propane, ethylene, or propylene have been mixed with the refrigerant gas in the second, i.e., lower temperature, stage in order to maintain low viscosity of the compressor oil. The foregoing hydrocarbons are generally very compatible and miscible in their liquid form with typical compressor oils of mineral oil and alkyl benzene types at low temperature. This compatibility and miscibility has helped prevent compressor oil from being frozen in the low temperature region.
Due to their flammable properties, use of these hydrocarbons in refrigeration equipment has become a public concern particularly with respect to safe transportation and product application. Flammable refrigerants require additional safety precautions, invite regulatory agency scrutiny, require costly flammability tests, may have negative customer perception, and as stated earlier may complicate transportation needs. There is therefore a need to redesign the low temperature stage of these ultra-low temperature refrigeration systems so that no hydrocarbons are utilized, thus eliminating flammable refrigerant additives while maintaining performance, reliability, agency listings, and manufacturability. A successful design of non-hydrocarbon ultra-low temperature refrigeration systems that allows for shipping and applying such systems in the field without conflicting with safety regulations is desired.
The present invention overcomes the need for using hydrocarbons in a refrigeration system by utilizing refrigerants R134a, R404a, and/or R508b. To achieve desired properties, these refrigerants may be used alone or in a xe2x80x9ccocktailxe2x80x9d mixture. For example, refrigerant R134a has an advantage of being a pure refrigerant but has a high deviation from R508b""s boiling point. Refrigerant R404a has an advantage of having a boiling point that is appropriate for less off-set from the boiling point of R508b but is a mixture of three HFCs (R125+R143a+R134a). Refrigerant R508b may be used at 100% level but may upset the energy balance of a cascade refrigeration system especially at a high ambient temperature level.
It is therefore a feature and advantage of the present invention to provide a non-hydrocarbon ultra-low temperature refrigeration system that can safely be transported and applied in the field as needed without the risks associated with hydrocarbon ultra-low temperature refrigeration systems and the like.
It is another feature and advantage of the present invention to provide a non-hydrocarbon ultra-low temperature refrigeration system that utilizes hydrofluorocarbon (HFC) refrigerants which expand the temperature glide between the bubble point and the dew point of a mixture. This temperature glide expansion allows the refrigerant liquid mixture to exist at much warmer temperatures in order to xe2x80x9cwetxe2x80x9d the evaporator interior surface for a flushing effect.
It is another feature and advantage of the present invention to provide a non-hydrocarbon ultra-low temperature refrigeration system which reduces the freezing point of the compressor oil and refrigerant mixture below a typical level for a two-stage cascade refrigeration system.
It is another feature and advantage of the present invention to provide a non-hydrocarbon ultra-low temperature refrigeration system that utilizes an oil separator in the low/second stage of the system which balances between the saturation level of the compressor oil in the refrigerant mixture and the efficiency of the oil separator. This oil separator prevents the low temperature refrigerant mixture from becoming overburdened by the oil residue from the compressor while it still achieves the necessary and desired performance for the ultra-low temperature refrigeration system.
There has been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purposes of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent construction insofar as they do not depart from the spirit and scope of the present invention.